How to get name of image corresponding to active paint slot?
Material.texture_paint_images contains all image references used for painting.

import bpy

mat = bpy.context.object.active_material

for idx, img in enumerate(mat.texture_paint_images):
    print (idx, img)
    if idx == mat.paint_active_slot:
        print ("Index:", idx)
        print ("Name:", img.name)


Even shorter (without the iteration):

&gt;&gt;&gt; mat = bpy.context.object.active_material
&gt;&gt;&gt; print (mat.texture_paint_images[mat.paint_active_slot].name)
'Material Diffuse Intensity'